Macaluso A, Best E A, Bender R A
Department of Biology, University of Michigan, Ann Arbor 48109.
J Bacteriol. 1990 Dec;172(12):7249-55. doi: 10.1128/jb.172.12.7249-7255.1990.
A positive, genetic selection against the activity of the nitrogen regulatory (NTR) system was used to isolate insertion mutations affecting nitrogen regulation in Klebsiella aerogenes. Two classes of mutation were obtained: those affecting the NTR system itself and leading to the loss of almost all nitrogen regulation, and those affecting the nac locus and leading to a loss of nitrogen regulation of a family of nitrogen-regulated enzymes. The set of these nac-dependent enzymes included histidase, glutamate dehydrogenase, glutamate synthase, proline oxidase, and urease. The enzymes shown to be nac independent included glutamine synthetase, asparaginase, tryptophan permease, nitrate reductase, the product of the nifLA operon, and perhaps nitrite reductase. The expression of the nac gene was itself highly nitrogen regulated, and this regulation was mediated by the NTR system. The loss of nitrogen regulation was found in each of the four insertion mutants studied, showing that loss of nitrogen regulation resulted from the absence of nac function rather than from an altered form of the nac gene product. Thus we propose two classes of nitrogen-regulated operons: in class I, the NTR system directly activates expression of the operon; in class II, the NTR system activates nac expression and the product(s) of the nac locus activates expression of the operon.
利用针对氮调节（NTR）系统活性的正向遗传选择来分离影响产气克雷伯菌氮调节的插入突变。获得了两类突变：一类影响NTR系统本身并导致几乎所有氮调节功能丧失，另一类影响nac基因座并导致一类氮调节酶的氮调节功能丧失。这些依赖nac的酶包括组氨酸酶、谷氨酸脱氢酶、谷氨酸合酶、脯氨酸氧化酶和脲酶。已证明不依赖nac的酶包括谷氨酰胺合成酶、天冬酰胺酶、色氨酸通透酶、硝酸还原酶、nifLA操纵子的产物，可能还有亚硝酸还原酶。nac基因的表达本身受到高度的氮调节，这种调节由NTR系统介导。在所研究的四个插入突变体中均发现了氮调节功能的丧失，这表明氮调节功能的丧失是由于nac功能缺失而非nac基因产物的改变形式所致。因此，我们提出两类氮调节操纵子：在I类中，NTR系统直接激活操纵子的表达；在II类中，NTR系统激活nac表达，nac基因座的产物激活操纵子的表达。
